1. Field of the Invention
This invention relates to an electronic still camera system using a frame transfer type charge-coupled device (CCD), and more particularly to an electronic still camera system in which signal charges for two fields per one picture frame can be read out of a frame transfer type charge-coupled device (CCD).
2. Description of the Background Art
A television camera with a frame transfer type charge-coupled device (hereinafter referred to as "an FTCCD", when applicable) is well known in the art. In the FTCCD, charge signals which are formed in correspondence to an image which is formed on a light receiving section thereof, are transferred, in a parallel mode, separately according to fields to a charge storing section. The charge signals are read out of the storing section according to the scanning order. Transferring the charge signals from the light receiving section to the charge storing section for the first field and transferring the signal charge from the light receiving section to the charge storing section for the second field are carried out alternately.
Accordingly, even if, in the case where such an FTCCD is used for a still camera, picture charge signals for one frame have been formed, the charge signals which can be delivered out are only those for one field. Therefore, the vertical resolution is decreased to half of that which is provided when it is used as a television camera. This drawback is serious especially for a still camera because the defects of pictures formed thereby attract attention.
As is well known in the art, there are available various charge-coupled devices having different picture signal transferring techniques, such as an interline transferring type charge-coupled image pickup device and a frame transferring type charge-coupled image pickup device. In the interline transferring type CCD, charges produced according to incident light are read out in an image pickup cell array comprising image pickup cells arranged two-dimensionally. In order to form picture signals according to a raster scanning system, the optical charges which are produced for one horizontal scanning line by the cells in the image pickup cell array are vertically transferred to horizontal transferring CCDs (hereinafter referred to as "HCCDs", when applicable) in a parallel mode. The charges in the HCCDs are transferred horizontally for every picture element, to provide picture signals. That is, it is necessary to perform the above-described vertical and horizontal transferring operations successively for all the horizontal scanning lines.
In the interline transferring type CCD, a picture can be formed with two fields per one picture frame. However, the construction is intricate, and the aperture efficiency is low. Therefore, the device is not suitable for high sensitivity and high picture element density. On the other hand, in the frame transferring type CCD, the light receiving section is integral with the charge transferring section, and therefore the above-described difficulties are eliminated. However, the device is still disadvantageous in that the one-picture two-field picture formation cannot be obtained because the vertical transferring operation is carried out. All conventional charge-coupled image pickup devices include such vertical transferring operation as described above.
On the other hand, in order to positively achieve the vertical transferring operation, each transferring region for one picture element is provided with four electrodes of polycrystalline silicon so that the vertical transferring CCDs (hereinafter referred to as "VCCDs", when applicable) are driven by four clock pulses. Accordingly on the substrate of the image pickup device, the circuit elements for one picture element occupy a relatively large area. Therefore, the density of the image pickup cells and accordingly that of the picture elements is low, and therefore the picture resolution power is degraded.